Microemulsion Polymerization. 2. Influence of Monomer Partitioning, Termination, and Diffusion Limitations on Polymerization Kinetics
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Macromolecules 2001, 34, 3233-3244 3233 Microemulsion Polymerization. 2. Influence of Monomer Partitioning, Termination, and Diffusion Limitations on Polymerization Kinetics Renko de Vries, Carlos C. Co, and Eric W. Kaler* Center for Molecular and Engineering Thermodynamics, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received July 18, 2000; Revised Manuscript Received January 2, 2001 ABSTRACT: We investigate the polymerization kinetics of microemulsions prepared with the cationic surfactant dodecyltrimethylammonium bromide and the hydrophobic monomers n-butyl methacrylate, tert-butyl methacrylate, n-hexyl methacrylate, and styrene. Our previous model for microemulsion polymerization kinetics cannot account for the kinetics of these systems. Using the results of small- angle neutron scattering monomer partitioning studies and an extended kinetic model to analyze the data, the failure of the original kinetic model is shown to be due to a combination of nonlinear monomer partitioning, nonnegligible bimolecular termination, and, in some cases, diffusion limitations to propaga- tion. 1. Introduction agreement with our experimental observations. More- over, full quantitative agreement between theory and In the first paper of this series we investigated the experiment was found for all conversions, using litera- thermodynamics of polymerizing microemulsions. The ture values for the various parameters. monomer concentration in polymer particles formed during microemulsion polymerization was determined Previously we have suggested that in some cases the failure of our simple kinetic model2 may be due to the using small-angle neutron scattering (SANS), and a 7 simple thermodynamic monomer partitioning model pH dependence of the initiation efficiency of unbuffered was developed that explains our experimental findings. persulfate initiators that are typically used. However, for styrene microemulsion polymerizations initiated These results set the stage for the present paper, the with the pH-insensitive cationic initiator 2,2′-azobis(2- aim of which is to develop a better understanding of the amidinopropane) hydrochloride (V50), it was found that kinetics of microemulsion polymerization of hydrophobic the rate maximum still occurs at ∼20%. Similarly, rate monomers initiated with water-soluble initiators. A maxima at conversions significantly below 39% have number of attempts have already been made at model- been found for a number of other systems initiated with ing the microemulsion polymerization kinetics for these 1-6 V50 and buffered persulfate initiators. Hence, while pH systems, but as yet there is no generally applicable effects may be important in some cases, they are not model. Models that have been proposed each address a the only cause of the observed differences. specific system. Instead, it is more likely that the failure of our An elaborate model for the kinetics of styrene micro- original kinetic model for systems other than the base emulsion polymerization has been proposed by Guo et case of C MA/DTAB/DDAB microemulsions is due to 5 6 al. This model accounts for the observed kinetics up to nonnegligible bimolecular termination, nonlinear mono- fairly high conversions but fails to describe the observed mer partitioning, or diffusion-limited propagation due growth of the particle number density. To account for to glass transition. In part 1 of this series, we have the experimentally observed kinetics, the model requires shown that monomer partitioning may be significantly that the rate constant for the capture of an aqueous nonlinear. However, as it turns out, nonlinear monomer phase free radical by a monomer-swollen micelle is partitioning alone is insufficient to explain the failure many orders of magnitude smaller than that for capture of our previous kinetic model for styrene microemulsion by a polymer particle. A very similar model was recently polymerizations. This suggests that termination and/ 6 proposed by Mendizabal et al. or diffusion limitations may also be important. We have previously studied the microemulsion po- Here, we study the polymerization kinetics of n-butyl lymerization of n-hexyl methacrylate (C MA) in micro- 6 methacrylate (nC4MA), tert-butylmethacrylate (tC4MA), emulsions prepared using a mixture of dodecyltrimeth- C6MA, and styrene microemulsions prepared with DTAB. ylammonium bromide (DTAB) and didodecyldimethyl- We have previously studied the monomer partitioning 1-4 ammonium bromide (DDAB) surfactants. A simple behavior of these four systems in part 1 of this series. 2 kinetic model was developed, and solved analytically, The choice of these monomers allows us to indepen- based on the simplifying assumptions that bimolecular dently probe the effects of monomer water solubility, termination is negligible and that the monomer con- polymer glass transition temperature, and propagation centration in the growing particles decreases linearly rate constant on the polymerization kinetics. We also with increasing conversion. With these assumptions, a extend our previous kinetic model by including both the rate maximum is predicted to occur at 39% conversion, observed nonlinear monomer partitioning and account- independent of any characteristic of the system, in good ing for the possibility of bimolecular termination. Using the extended kinetic model to analyze the data, we * Corresponding author. Tel (302) 831-3553; Fax (302) 831-8201; demonstrate that for nC4MA and C6MA microemulsions E-mail [email protected]. the failure of our previous kinetic model is due to 10.1021/ma001247j CCC: $20.00 © 2001 American Chemical Society Published on Web 04/10/2001 3234 de Vries et al. Macromolecules, Vol. 34, No. 10, 2001 Figure 2. Experimental and model calculated kinetics for C6- MA/DTAB microemulsions initiated with varying concentra- tions of V50: (O) 0.044 mM and (0) 0.015 mM that correspond Figure 1. Sample withdrawal system used in measuring respectively to 0.045 and 0.015 wt % relative to the amount of polymerization kinetics. In position A, the sample is with- monomer in the microemulsion. drawn directly into 20 mL ampules for gravimetric analysis. In position B, the residue in the sampling lines are flushed. sample as it enters the ampules, and freezing in liquid nitrogen is done only as a precaution. Even when kept at 60 °C for an additional 30 min, no additional polymerization can be de- nonlinear monomer partitioning. For styrene micro- tected in the sealed samples that have been exposed to oxygen. emulsions, we argue that bimolecular termination can- Condensate on the exterior of the ampules is washed off with not be neglected. In addition, diffusion limitations to acetone, and then the amount of sample in the ampules is propagation may be important for both tC4MA and measured by difference. Water and monomer are evaporated styrene microemulsion polymerizations even at low by positioning the ampules horizontally overnight under a conversions. gently blowing air stream. A volatile solvent (e.g., acetone) in We consider chemical initiation by water-soluble which the surfactant is insoluble is then used to extract initiators only. Initiation by oil-soluble initiators is more residual volatiles from the cake, and the ampules are air-dried again after rotating them 180° in the rack. After thorough air- complicated. The radical pair that presumably forms in drying, the samples are dried further in a vacuum oven either the micelles or in the polymer particles may initially kept at 30 °C for 12 h and then ramped up to 60 °C either recombine or the radicals may exit the micelles for 6 h. The samples are cooled to ambient temperature and or particles in a way that is expected to depend then reweighed. The weighings are performed carefully on a sensitively on the micelle and particle properties. This four-digit balance, and the precision of the 30-40 conversion complication is absent for water-soluble initiators. measurements obtained for each reaction is routinely better than (0.5%. The accuracy of these measurements is within 2. Materials and Methods (1% and is limited by the accuracy in preparing the original microemulsion. All monomers (nC4MA, tC4MA, C6MA, and styrene) were To extract rates of polymerization, the conversion vs time purchased from Scientific Polymer and vacuum distilled to data are differentiated using Craven and Wahba’s cross- remove inhibitors. The distilled monomers were stored in a validation smoothing spline algorithm.8 Statistical validity is freezer for less than 5 days before use. DTAB (99+%) from then examined using a bootstrapping algorithm9 to account TCI and V50 initiator (98.8%) provided by Wako were used for experimental errors in measuring both conversion and as received. Prior to preparing the microemulsions, deionized time. For each iteration of this bootstrapping procedure, all water (18.3 MΩ cm) was boiled under vacuum for at least 30 data points are resampled from normal distributions centered min, and the monomers were sparged with ultrahigh-purity around the original data points with standard deviations of N2 (<1 ppm of O2) for 15 min to remove oxygen and then 1 s and 0.5% respectively for the time and conversion. The sealed. Thirty grams of DTAB was weighed into a 500 mL resampled data set is then differentiated using the cross- water jacketed reactor equipped with a stirrer, condenser, and validation smoothing spline algorithm after which the rates heated reactor head. The reactor was sealed and then purged at a few randomly